The present invention generally relates to the field of interlaced video-material, and particularly to a method and a system for performing motion detection in an interlaced video sequence.
In terms of its structure, a video sequence can be classified into two types that are progressive scan video sequence and interlaced scan video sequence. A progressive scan video sequence is composed of a series of frames, wherein the lines of each frame are drawn in sequence, i.e. a frame in the sequence is refreshed sequentially line by line from the top to the button of the frame. In such progressive scan video sequences, a motion detection can be performed by comparing in a known manner the lines, pixels, or blocks of pixels of successive frames.
On the other hand, an interlaced scan video sequence is composed of a series of images that are referred to as fields. Fields are similar to frames but have only half the vertical resolution thereof and are generally displayed twice as fast. In fact, interlaced scanning halves the number of scan lines in each field of the image sequence, thereby discarding half the information necessary to define each image in the vertical direction fully. For example, all European television pictures or frames are composed of 575 scan lines. However, each frame is transmitted as two separate fields of 287 or 288 lines, one so-called odd field consisting of the odd-numbered lines and the following so-called even field comprising the even-numbered lines.
Several methods are known from the prior art for performing a motion detection in such an interlaced video material. One approach consists in using two consecutive fields of different raster, namely an even field and an odd field. The motion detection itself is done by comparing the pixel values of the two consecutive fields within a given area of the fields. However, this method does not enable an exact detection of moving areas or pixels because the two consecutive fields are defined by distinct scan lines and thus by distinct pixels.
According to an alternative approach, it is known to use two consecutive fields of the same type like two consecutive even fields or two consecutive odd fields. A motion detection can be achieved by calculating within the same area the absolute difference between the pixel values of the two selected fields. The absolute differences of each pixel in the area are then summed and compared to a predetermined threshold. For values below said threshold, it is assumed that there is a motion, while for values above said threshold, it is assumed that the area is static.
However, as the two selected consecutive fields of the same type are separated by a field of the other type, some movements occurring between the two selected fields may be ignored and considered as static, especially around the contours of an object. Additionally, in order to perform a continuous motion detection it is necessary to store three successive fields, such that more video memory is required.
Another alternative known approach for detecting movements in a video sequence combines the two above approaches and compares the content of three successive fields in time: one field of a given type encompassed by two fields of the other type. This so-called three-field technique is nevertheless more complicated, requires more video memory, and misdetection may still exist.